This invention relates to precipitation hardenable, nickel-iron base alloys, with or without cobalt, and articles made therefrom, that contain niobium, titanium, and silicon, and in particular, to such an alloy and article in which the elements are critically balanced to provide a unique combination of controlled thermal expansion and good elevated temperature tensile and stress rupture properties.
The high in-service temperatures to which controlled thermal expansion, high temperature nickel-iron base and nickel-cobalt-iron base alloys are exposed in use, are expected to become still higher. Furthermore, the requirements for the stress rupture and tensile properties of such alloys are becoming ever more stringent. Accordingly, a need has arisen for a high temperature, controlled thermal expansion alloy having better notch and combination smooth/notch stress rupture properties together with higher tensile strength and better ductility than the known high temperature, controlled thermal expansion alloys.
Furthermore, the age hardening heat treatments specified by the users of high temperature, controlled thermal expansion alloys are becoming shorter in duration and it is necessary that such Ni-Fe base and Ni-Co-Fe base alloys be capable of attaining the required strength within such shortened aging cycles.
The nickel-cobalt-iron base, precipitation strengthenable alloy disclosed in U.S. Pat. No. 4,006,011 ('011) issued to Muzyka et al. on Feb. 1, 1977 provides a good combination of very high room temperature tensile strength, about 145-150 ksi, together with a low coefficient of thermal expansion. In practice, however, it has been found that thermomechanical processing of the alloy can result in the mechanical properties of the wrought alloy being directional or anisotropic.
The alloy described in U.S. Pat. No. 4,200,459, issued to Smith, Jr. et al. on Apr. 29, 1980 sought to provide adequate notch rupture properties in a nickel-iron base, age-hardenable, controlled thermal expansion alloy without thermomechanical processing. To this end the patent describes complex relationships for balancing the hardener elements, Nb, Ti and Al with the other constituents. Although overaging of the alloy was recommended to benefit notch rupture ductility, the use of such overaging heat treatments results in markedly lower peak or short term tensile strength than was provided by the alloy of the '011 patent. It was found in practice that the precipitation of one or more secondary phases during such overaging heat treatments depleted the alloy of the primary strengthening phase.
U.S. Pat. No. 4,487,743 ('743) issued to Smith et al. on Dec. 11, 1984 relates to nickel-iron base and nickel-cobalt-iron base alloys which include silicon for the stated purpose of improving stress rupture notch strength without the necessity of an overaging heat treatment. Experience with the alloy of the '743 patent has shown that the stress rupture properties of the alloy are dependent on mechanical processing to a large degree and thus not consistently attainable, particularly when the alloy is exposed to very high temperature during manufacturing operations such as brazing, coating and others.